92 H. J. FLEURE. 



cteuidia of Ceplialopods are equivalent, for the Ceplialopods 

 have increased the efficiency of this branchial cavity by a 

 device of their own, which permits the retention of the 

 kidney and anal openings in their ancient position. The 

 early Gastropod had to adapt itself to a shore life, where the 

 branchial cavity was not as easily rinsed as hi the more 

 pelagic ancestor, and where, therefore, the excretory pro- 

 ducts tended to spread over and interfere with the efficiency 

 of the ctenidia, a tendency very imperfectly counteracted by 

 the appearance of the slit in the shell. It was therefore 

 desirable that any possible separation of incoming and 

 outgoing currents should be encouraged. The reduction of 

 its excretory function by the post-torsional left (pre-toi*sional 

 right) kidney promoted this kind of separation between the 

 incoming current of that side and the median outgoing 

 one, and thus made the ctenidium of this side the more 

 efficient. 



Probably long before this the pre-torsioual left side had be- 

 come more especially connected with the genital function, for 

 we find such a condition in practically all Ceplialopods and 

 Gastropods. As the genital products in the ancestral Gastro- 

 pods were expelled through the kidney, the renal opening of 

 this side was very important. This explains why it is that 

 when one kidney diminishes it is that of the pre-torsional 

 right (post-torsional left) side. 



In such a form slight clockwise variations of the position 

 of the branchial cavity would — 



(a) Place the more efficient ctenidium in a less suitable 

 position nearer the median line where the blockage of the 

 foot would be most felt. 



(h) Place this ctenidium at a lower level than the anus 

 and excretory openings, and thus make it likely to get soiled 

 and hampered by ffecal and excretory matter falling on it, 

 tliere being no such powerful outgoing clearing currents as 

 in Ceplialopods. 



Counter-clockwise variations would, on the other hand — 



(a) Place the more efficient ctenidium always in a better 



